Direct reduction iron (DRI) production has been developed to overcome difficulties (e.g. high capital expense, high pollution, and the need of coking coal) of conventional blast furnaces. Direct reduction typically uses shaft furnaces (such as the MIDREX® Process) or coal-fired rotary kilns. Rotary kiln production is limited because the kiln cannot be built larger than about 200,000 tons per year. Also, the use of lump ore and coal with high levels of ash and sulfur yields in low quality product.
The shaft furnace (SF) process uses a reducing gas or a reduction gas, which comprises a significant amount of carbon monoxide (CO) and hydrogen (H2) and a lesser amount of methane (CH4) and inert gases such as nitrogen. Iron ore is reduced in solid state at 800 to 1,050° C. (1,470 to 1,920° F.) by the reducing gas. The reducing gas flows up the furnace, heating the descending iron oxide to reduction temperatures. The hydrogen and carbon monoxide react with the oxygen in the iron oxide, yielding the reduced products. The reduction gas is conventionally made by reformation of natural gas, heated to a suitable temperature, and then fed into the shaft furnace where the direction reduction takes place. Natural gas serves both as the fuel and source of the reductant.
The spent reducing gas exits from the top of the shaft furnace, and is called the top gas (TG), which still contains a substantial amount of both CO and H2, and thus is usually recycled. The volume of the top gas does not vary for a given production rate. As much SF TG as needed to achieve the desired temperature is used as fuel gas for the reduction gas heater, then the balance is recycled back into the SF. Conventionally, the recycled top gas is mixed with fresh reduction gas (carbon monoxide, hydrogen, and lesser amounts of methane) made by reforming of natural gas or produced via coal gasification, and used again in the shaft furnace. To be recycled, the top gas must first be cleaned of solids and have its carbon dioxide content reduced. In order to remove carbon dioxide (CO2), the gas must also be cooled. The combined feed to the shaft furnace should have a CO2 content of 2-3 percent or less, which ensures that the reducing gas (containing the recycled top gas and fresh reducing gas, e.g. syngas from the gasification plant) has a sufficiently high reductants (H2+CO) to oxidants (H2O+CO2) ratio for efficient iron oxide reduction. The CO2 removal system will also remove the sulfur gases contained in the recycled top gas. The building and operation of suitable equipment components with sufficient capacity for top gas cleaning and CO2 removal represent a very significant capital and operational expenses in the direct reduction process, and it is desired that this expense be decreased as much as possible.
Not all of the top gas can be recycled in order to avoid the accumulation of inert gases in the shaft furnace reduction gas loop. Conventionally, this portion of the top gas not returning to the shaft is referred to as Top Gas Fuel (TGF), which is used as fuel in the process, to heat up the reduction gas introduced into the shaft furnace.
Because abundant, inexpensive natural gas is often not available in many locations, processes have been developed to use synthesis gas, or syngas, from gasification of coal, especially low grade coal and other low value carbonaceous fuel, as an alternative to reformed natural gas. U.S. Pat. No. 4,325,731 discloses a process in which the reducing gas is produced by reforming syngas from gasification. U.S. Pat. No. 4,046,555 provides another solution by adding relatively pure hydrogen to syngas to form a reducing gas, which includes a shift reaction to convert CO into CO2 and H2O. In U.S. Pat. No. 4,246,024, the reducing gas is produced by reforming a syngas within the same reactor where the direct reduction iron reactions take place. These processes all have high energy and capital costs and low efficiency.
A leading syngas production technology is the Synthesis Energy Systems Inc. (SES) fluidized bed gasification process, wherein solid feed stock, e.g. pulverized coal, is fed into a fluidized bed gasifier where it reacts with steam and oxygen or air, and is gasified to produce syngas which contains principally hydrogen, carbon monoxide, carbon dioxide. The syngas product also contains a lesser amount of methane, at concentrations appropriate for use as reducing gas in an iron reduction shaft furnace.
The syngas exiting the gasifier is hot, dirty, and contains a significant amount of non-reducing gas components. It is then cleaned, and conditioned to remove most of the undesired components, including some carbon dioxide.
Currently the syngas is also cooled and depressurized to about 3 barg in a turboexpander, which generates electricity.
There is a need to improve the energy efficiency of the conventional direct reduction iron production technology. There is also a need to improve the efficiency of using syngas produced from the coal gasification system.
The present invention provides improved processes involving the integration of two plants, which enable them to use the energy from both plants more efficiently.